If lets say, my scene have a lot of objects, if I only zoom in on a particular object and render, will the render be faster compared to me rendering everything because it only have to render that object and not everything since not the entire set is in the field of render?

1 more thing, when I render, max takes up 100% of my CPU. Is there a way I could limit its usage to like 90% so that I can do some less intensive work while its rendering like playing minesweepers and working on a spreadsheet?

edit:I heard some people say that mentalray could render faster than vray but still produce renders of almost the same quality as vray. Is that true?